This invention relates to accessories for power tools and in particular relates to an improved tool guide in the form of an overhead radial arm system or device adapted to co-operate with a portable machine tool so as to guide said tool over a workpiece.
The radial arm concept of machine tooling is a traditional technique where a track bearing rotatable arm is suspended above a workpiece. The arm is adapted for radial rotation through a horizontal plane and is provided with a machine tool which moves back and forth along the arms track. This type of tooling finds its most common application in the radial arm saw but the radial arm concept can be readily adapted to other machine tools, for example, a router.
Radial arm tools provide a highly versatile form of machining capable of cross cutting, ripping, shaping, compound miter cutting etc.
Many forms of radial arm machinery are available including a wide range of dedicated machines with permanently fitted machine tools; for example, saw heads. In addition, a range of radial arm xe2x80x9cdevicesxe2x80x9d are known which provide the skeleton of the machine fitted with a carriage that can mount a range of retro fittable machine tools including hand saws and routers.
A generally accepted limitation of such radial arm machines is a lack of precision caused by the elongate radial arm which must provide radial movement about a rearward pivot above a workpiece and in particular, the extended length of the arm and the distance the machine tool must move from the pivot point introduces many limitation to the accuracy of the set up. The length of the elongate arm and the pivoting means invariably results in the radial arm failing to achieve a fully rigid position particularly when fixed for a given sweep of the machine tool which may be required to traverse the length of the arm at an accurate height above the workpiece.
A further limitation of prior art radial arm machines is the adaptation of such machines to guide the machine tool along the length of the radial arm track as the principle mode of action with the radial arm fixed and locked at one particular radial angle.
For these reasons all prior art machines lack sufficiently accurate rearward pivot systems to allow the radial arm tracks to move in a radial sweeping motion so as to allow a machine tool to radially sweep across a workpiece surface, particularly where surface dressing of large workpieces is required.
Accordingly, in one aspect the invention provides a guide for a machine tool including a vertical upper column adapted for stable positioning at a work station and an elongate horizontally orientated arm extending outwardly from and pivotally attached to said upper column wherein said pivot is provided by at two vertically spaced apart pivot points so as to allow said arm free radial or arcuate movement in a horizontal plane.
The pivot points may be positioned outward of the central axis of said upper column. The outward position may be provided by outposts fitted to said column and vertically spaced apart.
The horizontal arm may be fitted to said pivot points by way of an elongate pivot column to provide clear rotation in a horizontal plane and reduced movement in a vertical plane.
The upper column may be vertically adjustable up and down providing a vertical adjustment of said horizontal arm relative to said workstation.
The column may telescopically co-operate with a stand means to allow said horizontal arm to be adjusted vertically relative to said workstation. The upper column may be adapted for snug telescopical fit within a stand column and provided with teflon or other lubricating means to ensure smooth co-axial movement between the upper column and the stand.
The column may be of square channel cross section to prevent radial co-axial movement but allow linear co-axial movement between the upper column and the stand column.
The liner co-axial movement may be effected by an hydraulic ram fitted to said stand and which acts on said upper column.
The horizontal arm may be provided with a carriage for mounting said machine tool and a plurality of calibrated stops along the length thereof.
The carriage may include a transmission housing incorporating roller bearings and shaped guides to ensure accurate movement of said carriage during tracking along said arm. The elongate arm may be,of square channel section mounted on the diagonal so as to provide a track for said carriage with the carriage self seating on said track.
The carriage may include a locking pin adapted to selectively engage the calibrated stops along the length of said arm so as to lock said carriage.
The horizontal arm may be provided with pivot lock to lock said arm in a selected radial position.
In another aspect the invention provides a method of surface dressing a workpiece including mounting said workpiece under the elongate arm of the guide as previously described, providing said guide with a router or like machine mounted to the carriage and passing the operating machine tool over the surface of said workpiece in a series of radial sweeps to surface dress said workpiece. The workpiece in question may be wood or the like.